A turbine, or compressor, of a gas turbine engine comprises one or more stages of rotor blades arranged alternately with one or more stages of stator vanes. Each of the rotor blades comprises a root, a shank, a platform and an aerofoil. The rotor blades are arranged circumferentially around a rotor and the rotor blades extend generally radially from the rotor. The roots of the rotor blades are located in axially, or circumferentially, extending slots in the periphery of a rotor. The platforms of the rotor blades together define the inner boundary of a portion of the flow path through the turbine, or compressor. The rotor blades have tips at their radially outer ends. A stator casing surrounds the rotor blades and a liner is supported from the casing to define the outer boundary of the flow path through the turbine, or compressor. The liner is spaced radially from the tips of the rotor blades to define a clearance, or sealing gap.
The clearance controls the amount of gas, or air, leaking from the flow path through the turbine, or compressor. The gas, or air, leaking from the flow path flows through the clearance between the tips of the rotor blades and the liner rather than flow through the passage defined between adjacent rotor blades and the liner.
The casing of the stator may be provided with optical probes, capacitance probes, inductance probes or microwave probes located on the casing and arranged to measure the clearance between the rotor blade tips and the liner.
The optical probes are the most accurate but these are subject to the build up of dirt on the optical probe, which reduces the strength of the optical signal and may eventually lead to the loss of an optical signal. Capacitance probes are not as accurate or as robust.
These probes make measurements relative to a fixed datum, either on the casing or at another point behind the liner. This requires knowledge of the offset distance between the datum point and the inner surface of the liner. The offset distance changes as a result of wear of the liner, movement of the liner segments or thermal expansion of the liner segments etc.
In addition, the liner may be moved radially to actively control the clearance between the tips of the rotor blades and the liner.